This invention concerns a technique for producing contact lenses by molding the front and rear surfaces of the lens with one molding step. Two mold halves, the use of which is well known in the art, are used. A first mold half has a convex lens molding surface, and a second mold half has a concave lens molding surface. Contact lens forming material, e.g. monomer or a mixture of monomers suitable for making a contact lens, is placed on the concave surface. Upon mating of the mold halves, a contact-lens-shaped cavity filled with lens forming material is formed between the convex and concave surfaces. The lens forming material is then cured, e.g. polymerized, to form a contact lens.
In order to assure that the contact lens cavity is completely filled with lens forming material, extra material must be used. The excess material will, of course, be located outside of the lens cavity when the mold halves are mated. Like the material in the cavity, the excess material will also solidify during cure. This excess solid material so formed is called "flash." The flash may be softer and tackier than the cured lens, since it is in contact with air, a polymerization retarder, outside the cavity. This flash is potentially troublesome, because it can settle and stick to the molded lens, thereby rendering the lens unusable. It is conventional to provide an annular cavity surrounding the lens forming cavity to accommodate the flash. However, even when there is a flash cavity, the flash can still be troublesome.
Hamilton et al, in UK Patent Application GB 2 226 272A, disclose use of holes in the bottom of the flash cavity for draining the excess lens forming material. However the quantity of excess lens forming material is not large enough to readily flow by gravity through a small opening. Moreover the lens forming material has a tendency to wet the material of which the mold is constructed, making efficient removal by gravity flow unlikely.